1. Field of the Invention
The present invention relates to a thermosensitive recording material on which colored images are formed by heating. More particularly, the present invention relates to a thermosensitive recording material capable of forming thereon colored images resistant to fading and thus exhibiting a high degree of persistency during extended storage thereof.
The thermosensitive recording material of the present invention has a high whiteness and is capable of recording thereon colored images exhibiting an excellent resistance to moisture, heat, oily and fatty substances, and plasticizers, and thus has superior persistency when stored over a long period of time, and therefore is useful as colored image-recording sheets, sheets for use in facsimiles, word processors, CRT image printers and cash dispensers, as passenger tickets, commuter passes, labels such as POS labels, cards such as prepaid cards, and as transit passes.
2. Description of the Related Arts
It is known that a conventional thermosensitive recording material comprises a supporting substrate, for example, a paper sheet, synthetic paper sheet, or plastic resin film and a thermosensitive colored image-forming layer formed on a surface of the supporting substrate and comprising an electron-donative dye precursor, for example, a leuco basic dye, an electron-acceptive color-developing agent consisting of an organic acid substance, for example, a phenolic compound, and a binder. When the thermosensitive colored image-forming layer is heated imagewise, colored images are recorded thereon by a reaction of the dye precursor with the color-developing agent.
This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication Nos. 43-4,160 and 45-14,039 and Japanese Unexamined Patent Publication No. 48-27,736, and is widely employed in practice.
Namely, the thermosensitive recording material is advantageous in that colored images can be easily formed by heating alone, and the recording apparatus can be made compact and small in size, has a relatively low 10 price, and can be easily maintained. Therefore, this type of thermosensitive recording material is appreciated as a useful information-recording material for recording outputs of printers used with, for example, computers, facsimile machines, automatic ticket-vending machines, scientific measurement recorders, and CRT medical measurement recorders.
Nevertheless, the conventional dye-forming type thermosensitive recording materials in which the thermosensitive colored image-forming layer comprises a conventional color-developing agent together with the dye precursor and the binder is disadvantageous in that the resultant colored images fade with the lapse of time, presumably because of a reversible reaction of the dye precursor with the color-developing agent. This fading of the colored images is accelerated by exposure to light, high temperatures, and high humidity and is specifically promoted by contact with an oily or fatty substance or a plasticizer, to such an extent that the faded images cannot be recognized.
Many attempts have been made to retard or inhibit the fading of the colored images formed on a conventional thermosensitive colored image-forming layer containing a substantially colorless dye precursor comprising a lactone ring compound. For example, Japanese Unexamined Patent Publication Nos. 60-78,782, 59-167,292, 59-114,096 and 59-93,387 disclose a thermosensitive colored image-forming layer containing a phenolic antioxidant.
Japanese Unexamined Patent Publication No. 56-146,794 discloses a protective layer formed from a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer.
Japanese Unexamined Patent Publication No. 62-19,485 discloses that a compound having a certain chemical structure similar to that of the present invention is usable as a material for mainly pressure-sensitive recording paper sheets.
In the thermosensitive colored image-forming layer containing the phenolic antioxidant, the improvement effect of the phenolic antioxidant in the resistance to heat and moisture is not satisfactory. Also, the phenolic antioxidant, sometimes, undesirably causes the resultant colored image-forming layer to exhibit a low degree of whiteness.
When the protective layer or the intermediate and top layers are formed on the thermosensitive colored image-forming layer, the resultant colored images exhibit a significantly enhanced resistance to environmental conditions for a short period of time. Nevertheless, when the colored images are exposed to the environmental conditions for a long period of time, the fading of the colored images cannot be avoided. Also, in this type of recording material since a heat-insulating layer is formed on a surface of the colored image-forming layer, the resultant colored image-forming layer unavoidably exhibits a lowered degree of thermosensitivity during recording.
The compound disclosed in Japanese Unexamined Patent Publication No. 62-19,485 and having a certain chemical structure similar to that of the present invention, exhibits a color-developing activity equal to or lower than that of the conventional phenolic color-developing compounds. This fact will be illustrated in Comparative Examples 2 and 3 hereinafter.